Mooring arrangement

ABSTRACT

Arrangement for mooring, loading and unloading of a vessel, comprising: a stationary inner tower with a lower end fixedly anchored to the seabed, from where the inner tower extends upwards through the sea to an upper end over the sea level, which inner tower at level close to the seabed has through connections for hoses and cables for transfer of load and signals, which hoses and cables are brought further up through the inner tower and out of its upper end; a yoke that in one end is rotatably fastened to the inner tower, wherefrom the yoke extends further outwards to at least one outer ballastable end wherefrom moorings are arranged to keep the vessel anchored, on which vessel devices are provided to connect the vessel with the moorings and said hoses and cables for transfer of load and signals. The mooring arrangement is distinguished in that it further is comprising an outer tower with rotatable fastening to the inner tower, which outer tower from the fastening to the inner tower extends upwards outside the inner tower to a level over the upper end of the inner tower, wherein the rotatable fastening is placed below sea level and also is comprising the fastening of the yoke, such that the outer tower and the yoke as one unit is freely rotatable over and around the inner tower that is stationary anchored to the seabed; and a swivel provided between the upper end of the inner tower and the upper end of the outer tower, for rotatable transfer of load and signals with said hoses and cables between the inner and the outer tower and therefrom further to the vessel.

FIELD OF THE INVENTION

The present invention regards arrangements for mooring, loading andunloading of a vessel. The vessel is for example a FPSO-vessel(“Floating Production Storage and Offloading”), but can be a tanker oranother type of vessel. The load is preferably comprising hydrocarbonsin form of oil or gas, but can be any fluid or for example powder thatcan be transported through pipes. The invention is in particularrelevant for mooring, loading and unloading of a vessel of the FPSO-typeat shallow waters.

BACKGROUND OF THE INVENTION AND PRIOR ART

For mooring, loading and unloading of a vessel at shallow waters, forexample a vessel of the FPSO-type, it has proved difficult to arrangeanchor chains with sufficient slack such that a suitable resilience isachieved in the mooring system. At too low resilience in the mooringsystem, sudden vessel movements may result in too large forces. Anothercrucial factor regarding mooring is to achieve a position restoringforce in the mooring, such that the vessel as moored can lay in a stableposition during loading and unloading without occurrence of too strongforces.

Regarding conventional single anchoring systems with a floating buoyconnected with a chain or a connection arm to a foundation structurefastened to the seabed, it has proved that the position restoring forcesare insufficient.

Completely submerged systems for mooring and transfer of load andsignals are possible, but significant problems appear in that criticalcomponents are submerged in the sea. This shortens the lifetimesignificantly and results in severe problems with respect to repair andmaintenance.

In view of the above, mooring, loading and unloading at shallow watersare preferably undertaken by use of a mooring tower that extends fromthe seabed and up to above sea level, with a sufficiently resilientanchoring of the vessel to the top of the mooring tower.

Patent publication U.S. Pat. No. 4,516,942 contains description of amooring tower that extends from the anchoring at the seabed to over thesea level, where the tower is provided with a triaxial rotatable yoke inthe top in the mooring tower, for mooring and transfer of fluid andsignals. The yoke's fastening in the top of the mooring tower results inthat the mooring tower is subject to a very large momentum from themooring forces, and the dimensioning of the mooring tower is thereforevery powerful. Also the vessel that is to be moored to the mooring toweraccording to U.S. Pat. No. 4,516,942 must have a powerful dimensioningin order to handle the forces involved, in particular since the mainstructure of the vessel has to take up forces from the yoke from abovedeck level. The volume of the mooring tower and the yoke above the waterline and down to the deepest draught for vessels in the waters is verysignificant, resulting in risk for collision and heavy impact from ice,current, waves and wind. As apparent from the above a number ofdisadvantages are associated with the mooring tower according to U.S.Pat. No. 4,516,942.

The objective of the present invention is to provide an arrangement formooring without the disadvantages associated with the mooring toweraccording to patent U.S. Pat. No. 4,516,942 and other relevant priorart, as referred to above.

SUMMARY OF THE INVENTION

The objective of the present invention is met in that a mooringarrangement is provided, having design and distinguishing features asapparent from claim 1.

With the arrangement for mooring according to the present invention themooring forces providing momentum on the mooring tower are significantlyreduced, with resulting consequences for the dimensioning of the mooringtower and the anchoring to the seabed. The momentum of the mooringforces on the mooring tower can be zero at particularly preferredembodiments involving that the resultant of the mooring forces is inline through the anchoring in the seabed. With the arrangement formooring according to the present invention also the risk for collisionwith the vessel that is to be moored is reduced, in particular byplacing the yoke lower than the maximum draught of the vessel. Thereduced dimensions of the mooring tower are reducing the impact fromice, current, waves and wind.

DRAWINGS

FIG. 1 shows a side view of the arrangement for mooring according to theinvention, with a FPSO-vessel as moored.

FIG. 2 shows a top view of FIG. 1,

FIG. 3 shows an enlarged section of the mooring tower illustrated onFIG. 1,

FIG. 4 is a process diagram for the mooring arrangement.

DETAILED DESCRIPTION

In the following a particular embodiment is further described withreference to the drawings. In this connection only mooring, loading andunloading with a typical FPSO-vessel is considered.

FIG. 1 illustrates an embodiment of the arrangement according to theinvention, with a FPSO-vessel as anchored, whereby the arrangement isillustrated for a water depth of 25 m and with a FPSO with storagecapacity of about 1000000 bbl (160000 m³). The draught of the vessel is8.0 m with ballast and 15.0 m fully loaded, the length is 265 m and thebreadth is 42.5 m. This is of course only for illustration and scalingto other dimensions or vessels can be undertaken according to demand.

The arrangement according to the invention is in general most useful forwater depths from 20 m to 50 m, but the limits are not fixed. Thearrangement is completely making use of known components that all arewell proven and have exhibited long service life without need formaintenance. Yet the arrangement and all critical components aredesigned such that inspection is simple and replacement of criticalcomponents is possible. The design life will be chosen to a minimum of15 years for the illustrated embodiment.

FIG. 1 shows, as mentioned, a typical FPSO laying moored to thearrangement according to the invention, with hoses and mooringsconnected to the buoy of the vessel. The hoses for transfer of load,including cables for transfer of signals, extend out from the vessel viaa bending restrictor and hang as catenary lines towards a correspondingbending restrictor at the top of the outer tower of the arrangementaccording to the invention. The vessel is connected via a mooringfastening structure in the buoy with moorings to the ends of a yoke thatis placed submerged under water. As illustrated on the figure the yokeis fastened to the inner tower at a height of 7.0 m over the seabed. Thevessel can rotate freely around the inner tower with the yoke, the outertower, the moorings, hoses and cables.

The position restoring effect of the mooring arrangement is increased byballast provided between the outer arms of the yoke. In the illustratedembodiment the ballast is a 25 m long cylinder of diameter 3 m, dividedinto three compartments. After installation the central compartment ofthe ballast cylinder is water filled while the two side compartments arefilled with heavy slurry to provide suitable pretensioning of themoorings.

Between the top of the outer tower and top of the inner tower a swivelconnection is provided to achieve rotatable transfer of load andsignals. The swivel deck on FIG. 1 is at elevation 14 m. The swivelcomprises a process swivel, a swivel for electrical power and electricaland optical signals, and a swivel for the hydraulics. All seals aredesigned as double barrier seals. Means will be provided for inert gaspurging or similar for rooms with swivels, for pressure monitoring andpressure control. Also emergency shut-down systems for fluids andelectrical signals will be provided.

The number of hoses and cables is in principle without any particularlimit. When the expression hoses here is used it is considered to meanboth stiff pipes and flexible pipes, except from the distance betweenthe top of the outer tower and the vessel where hoses or pipes have tobe flexible because they are hanging as catenary lines above the waterlevel.

A typical tower will for example comprise two 12 inch hoses or pipes forloading of crude oil, a 12 inch flexible pipe for water injection, andan additional reserve pipe of 12 inch. The number, the types anddimension of pipes, hoses or cables can of course be varied according tothe demand. As illustrated on FIG. 1, and appearing more clearly on FIG.3, the inner of the mooring tower is open to the atmosphere and is dryfor personnel access. The view of FIG. 1 and FIG. 2 as combinedillustrates both the design of the yoke and the inner tower. The innertower is fixedly fastened to the seabed. Three suction anchors areillustrated in triangular configuration with 15 m between each suctionanchor, wherein each suction anchor, having a diameter of 8 m, is topenetrate 8 m into the seabed. The design as illustrated is consideredto be sufficiently powerful to hold a FPSO of the specified sizepassively anchored in a 100 years condition of harsh weather, wherebythe vessel is without available motor power. However, it is possible touse other types of anchoring to the seabed, provided that thedimensioning is according to the acting forces. Suction anchors arepreferable with respect to installation, fastening force with respect tovolume, difficult soil conditions and possible removal after use.Suction anchors can relatively easy be installed and later be removed.

The outer tower with all connected equipment and the yoke canweathervane freely with the vessel around the stationary inner towerfixedly anchored to the seabed, under all weather conditions.

The arrangement according to the present invention further comprisesequipment over the sea-line or placed dry within the tower, whichequipment is easily accessible for inspection and maintenance, and saidequipment is preferably of known type for rotatable communication offluid and signals, and mechanical transfer. The tower structure providesdry placement of said equipment and supports the connections for loadtransfer and signal transfer, which are hanging as catenary lines about20 m over the sea level.

With the arrangement according to the present invention the anchoringforces are transferred via the yoke to the seabed via the rotatablefastening, which in the illustrated embodiment is a rotatable disc. Therotatable fastening of the yoke and the outer tower is the only criticalcomponent that is not placed dry in the arrangement according to thepresent invention.

Some further details regarding some of the components are as follows:

The inner tower with mounted rotatable disc over a fast anchoring to theseabed represents the stationary part of the arrangement according tothe invention and supports all the equipment above. The inner towertakes up the forces acting on the outer tower because of ice, wind,current, the hanged-up hoses, etc., which forces are transferred fromthe outer tower via bearings along the length between the towers. Thebearings can be designed with sliding surfaces manufactured from Inconelcladded steel. The diameter on the lower part of the inner tower belowthe rotatable disc is according to the illustrated embodiment 3.7 m,while the diameter of the upper part of the inner tower is 3.1 m.

On the outside of the outer tower, at the water-line level, anice-breaking device is installed.

Disconnection of the whole mooring arrangement is possible within 48hours, for example at very severe ice conditions. The arrangement is,however, designed to withstand the worst 100 years conditions of ice.

A footbridge can be installed between the vessel and the towerstructure, for use at moderate or better weather conditions.

The rotatable disc has the outer tower mounted with bolts. The rotatabledisc has a protrusion to which the yoke is fastened The rotatable discis preferably of a well known type according to the so called“STL/STP-system”. The bearing means comprises a main radial bearingencompassing the inner tower and an upper axial bearing and a loweraxial bearing. Preferably a self lubricating bearing system of the typeOiles 500 is used, with bronze alloy bearings with PTFE-lubrication,chosen for long service life. The bearings are fixed to the rotatabledisc. The upper bearing ring supporting the upper axial bearing islocked to the stationary inner tower by use of a segment locking systemfastened in a circular groove on the centre shaft. This is a well provensolution used with good results for swivels subsea, according to theSAL-systems.

The fluid transfer system is also illustrated on FIG. 4 where a typicalprocess flow diagram is indicated. As apparent, means are also providedfor pigging of the load transferring lines and the water injection hose.

Installation and deinstallation is relatively simple because of thesuction anchors and large compartments that can be filled with air orballasted in a controlled way, and by installing the yoke afterinstallation of the tower structure. Then pipes and hoses are installed.It is estimated that 7-8 m water depth is required in order to tow atower structure as the one illustrated, when it is floating by use ofsuitable air filling. Preferably a crane vessel, divers and remotelyoperated vehicles are used in connection with installation anddeinstallation.

1. An arrangement for mooring, loading and unloading of a vessel,comprising: a stationary inner tower with a lower end fixedly anchoredto the seabed, from where the inner tower extends upwards through thesea to an upper end over the sea level, which inner tower at level closeto the seabed has through connections for hoses and cables for transferof load and signals, the hoses and cables brought further up through theinner tower and out the upper end, a yoke having one end rotatablyfastened to the inner tower, wherefrom the yoke extends further outwardsto at least one outer ballastable end wherefrom moorings are arranged tokeep the vessel anchored, on which vessel devices are provided toconnect the vessel with the moorings and said hoses and cables fortransfer of load and signals, an outer tower having a rotatablefastening to the inner tower, the outer tower from the fastening to theinner tower extending upwards outside the inner tower to a level overthe upper end of the inner tower, the rotatable fastening placed belowsea level and further comprising the fastening of the yoke, such thatthe outer tower and yoke as one unit is freely rotatable over and aroundthe inner tower that is stationary anchored to the seabed, and a swivelprovided between the upper end of the inner tower and the upper end ofthe outer tower for rotatable transfer of load and signals with thehoses and cables between the inner tower, the outer tower and thevessel.
 2. The mooring arrangement according to claim 1 furthercomprising the rotatable fastening of the outer tower and the yokelocated at a depth level deeper than the largest draught for the vesselproximate the lower end of the inner tower and a short distance over theseabed.
 3. The mooring arrangement according to claim 1 furthercomprising the rotatable fastening of the outer tower and the yoke, theplacement thereof, the length of the yoke and the length of the mooringswith the vessel anchored for loading and unloading, dimensioned suchthat an extension of the longitudinal axis of the yoke penetrates thestationary anchoring in the seabed.
 4. The mooring arrangement accordingto claim 1 wherein, the rotatable fastening of the outer tower and theyoke further comprises a rotatable disc with fixed self lubricatingbearings having a main radial bearing, an upper axial bearing and alower axial bearing.
 5. The mooring arrangement according to claim 1further comprising the yoke formed as a triangle with a top pointfastened to the inner tower and two outer ends between which outer endsballast chambers are provided and from which two outer end moorings areprovided to hold the vessel anchored.
 6. The mooring arrangementaccording to claim 1, further comprising the yoke fastened to aprotrusion in a rotatable disc that is rotatable around the inner tower,the yoke rotatably bolted to the protrusion with a fastening bolt with alongitudinal axis parallel with the plane of the rotatable disc andtangential to the rotation axis of the rotatable disc, and wherein theyoke includes a rotatable pin provided outside the rotatable disc in thelongitudinal axis of the yoke for rotation around the longitudinal axis,such that the yoke is moveable around three axes.